This disclosure relates generally to optical modulators, and in particular but not exclusively, relates to high-speed nanocrystal based clectro-optic modulators.
Electro-optic modulators employ electric fields to manipulate light within their constituent parts and are widely used in optical data transfer and processing. Two different types of known electro-optic modulators include electro-refraction modulators and electro-absorption modulators. Electro-refraction modulators rely on changes in the index of refraction of a material induced by an applied electric field to modulate the propagation of light through the modulator. One example of an electro-refraction modulator is based on a Mach-Zehnder interferometer. An incident light beam is split into two beams that propagate through the device on different paths and are subsequently recombined. An applied electric field alters the refractive index of the material along one of the paths to produce constructive or destructive interference when the beams are subsequently recombined.
Electro-absorption modulators achieve the desired light modulation by modifiying the light absorbing properties of a material with an electric field. Materials comprising multiple quantum well (xe2x80x9cMQWxe2x80x9d) structures are particularly suitable for use in such devices because the quantum confined Stark effect (xe2x80x9cQCSExe2x80x9d) allows the energy levels of the material to be shifted by significantly lower voltages than those required for most other materials. These shifts can be used to alter the light absorbing properties of a MQW-based electro-optic modulator at the wavelength of interest.
The QCSE has given rise to several innovations in electro-optic modulators. Electro-optic modulators employing QCSE have many applications in communications and special purpose computer systems. QCSE is a phenomenon which arises when an electric field is applied across the plane of heterostructure superlattices. In a quantum well at zero electric field, the electron and hole energy levels are defined by the well width, and the electrons and holes are strongly confined within the well layer. However, when an electric field is applied, the electrons and holes are moved apart and their energies are altered. This has the effect of shifting the absorption resonance, as well as, modulating the strength of absorption. This occurs because direct optical absorption of a photon above the band gap energy involves raising an electron from the valence band and putting it in the conduction band, otherwise known as the formation of an exciton (electron-hole pair).
Research is ongoing to find lower voltage, higher frequency, and more efficient electro-optic modulators. In particularly, electro-optic modulators that can be integrated with silicon semiconductor devices are of particular interest. By implementing enhanced electo-optic modulators in silicon, designers can incorporate optical functionality into mature, low cost ultra large-scale integration (xe2x80x9cULSIxe2x80x9d) technologies.